Support clnt_raw's use of FD_SETSIZE as a fake file descriptor.

[brooks]

Summary

Accomplish this by allocating space for it in __svc_xports and allowing
it to be registered. The failure to allocate space was causing an out of
bounds read in svc_getreq_common(). The failure to register caused PR 211804.

The bug was found with CHERI bounds checking.

PR: 211804
Obtained from: CheriBSD
Sponsored by: DARPA, AFRL

[brooks]

FWIW, NetBSD switched to using -1 instead of FD_SETSIZE to allow larger sets. We should probably pick up their assorted RPC fixes at some point.

[ngie]

In D10528#218125, @brooks wrote:

FWIW, NetBSD switched to using -1 instead of FD_SETSIZE to allow larger sets. We should probably pick up their assorted RPC fixes at some point.

+1

Does supporting code need to be added to not leak the extra bogus descriptor?

lib/libc/rpc/svc.c

116 ↗

(On Diff #27800)

This really should be 0, not '\0'.

[brooks]

• Also deregister fake sockets for raw RPC.